The present application relates to a light-transmissive optical sheet typically used as a functional film for liquid crystal display devices, and a backlight device and a liquid crystal display device provided with the same.
Liquid crystal display devices (LCD) have larger potentials of reduction in power consumption, downsizing and thinning as compared with cathode ray tubes (CRT), and those of a variety of sizes are widely applied at present to small-sized instruments such as mobile phones and digital cameras, up to large-sized liquid crystal television sets. Under such situation, the liquid crystal display devices are required for further improvements in the luminance, view angle, rapidness in the response, and fineness in the definition.
The liquid crystal display devices are classified into those of transmission type and reflection type. In particular, the transmission type liquid crystal display device has a liquid crystal display panel composed of a liquid crystal layer held between a pair of transparent substrates and a backlight unit as a light source for illumination. The backlight unit is classified into those of direct backlight type having a light source disposed directly next to the liquid crystal display panel, and those of edge-light type.
In the backlight unit used for the liquid crystal display devices, optical sheets or films such as beam-condensing sheets including prism sheets or lens sheets aimed at aligning the direction of emission of light from the light source to the front direction, and diffusion sheets aimed at improving uniformity in the luminance of light from the light source (see Japanese Patent Application Publication No. 2006-72249 (Document 1)).
FIG. 8 shows a general back-light unit for liquid crystal display devices. In FIG. 8, reference numeral 101 represents a light source, 102 represents a reflective plate, 103 represents a prism sheet, and 104 represents a diffusion sheet. The prism sheet 103 is composed of a large number of prisms 103a having a triangular section, arranged on a light emitting side. The prism sheet 103 condenses beam from the light source 101 coming thereinto and beam reflected on the reflective plate 102 in the front direction, by allowing them to refract at the slope of the prism 103a of the prism and transmit therethrough. On the other hand, the diffusion sheet 104 diffuses beam once condensed by the prism sheet 103 over a wide angular range, and allows it to emit so as to uniformalize the luminance.
The related art prism sheet is fabricated by forming a prism sheet on the surface of a base of a predetermined thickness, by integrated forming or transfer forming. The integrated forming includes hot press process using PET (polyethylene terephthalate) or PC (polycarbonate) resin, and fusion-extrusion molding. The transfer forming is generally such as the one allowing a prism layer, typically composed of an ultraviolet curing resin, to cure and to thereby adhere to the resin sheets.
For example, Japanese Patent Application Publication No. Hei 06-102507 (Document 2) describes improvement in the luminance of the panel in the front view, by disposing, between the light source and the liquid crystal panel, a light-transmissive lens film having on one main surface thereof a large number of prisms arranged in parallel, and having a smooth surface on the other main surface, so as to allow the beam emitted from the light source to align in the direction of the normal line of the liquid crystal panel. Document 2 also describes adjustment of the pitch of the prism on the lens film to 100 μm or smaller, so as to prevent moire fringes caused by interference with the pixel pitch on the panel.
In the lens film described in the above, the action of condensing the beam from the light source into the direction of the normal line of the liquid crystal panel is exhibited on the slope of the prisms formed on the lens film, and theoretically independent of the prism pitch. It is, however, very difficult in practice to stably form the apex and the basic angles of the prism appearing as complete edges over a long period, in the process of manufacturing. Thus, the above-described apex and the basic angles tend to be rounded to some degree so that the rounded portion does not contribute to improvement in the front luminance. As a consequence, any effort of preventing the moire fringes by narrowing the prism pitch of the lens film will result in increase in the rounded portion, and fails in obtaining a desired level of larger ratio of enhancement in the luminance. Also for a case where the rounded portion is intentionally formed to the apex, any effort of preventing the moire fringes by narrowing the prism pitch of the lens film similarly fails in obtaining a desired level of enhancement in the luminance.
Japanese Patent Application Publication No. Hei 06-102506 (Patent Document 3) describes prevention of the moire fringes ascribable to interference between a large number of prisms formed there and the pixel pitch of the liquid crystal panel, as well as improvement in the luminance of the panel in the front view, by disposing, between the light source and the liquid crystal panel, a light-transmissive lens film having on one main surface thereof a large number of prisms arranged in parallel, and having on the other main surface thereof a smooth surface, and by further disposing a diffusion film between the lens film and the liquid crystal panel.
Disposition of the diffusion film between the lens film and the liquid crystal panel, however, occasionally fails in obtaining a desired level of enhancement in the luminance, depending on characteristics of the diffusion film, because the beam condensed by the prisms cannot effectively be used.
Known methods of manufacturing the above-described lens film include those based on (1) die casting, (2) hot pressing, (3) UV process, and (4) extrusion molding of a thermoplastic resin, among which those based on (3) UV process and (4) extrusion molding are advantageous in view of productivity, and extrusion molding is still more advantageous in terms of production speed. From the view point of cost, the extrusion molding process, in need of only an inexpensive thermoplastic resin, is understood as being the best method of manufacturing, because the UV process suffers from expensiveness of ultraviolet curing resins (UV resins) as well as necessity of using an expensive film such as polyethylene terephthalate (PET) film as a base.
In the field of liquid crystal TV in recent years, there have been increasing trends in expanding the screen size, and this consequently demands increase in the size of the optical sheets represented by the prism sheet.
The optical sheets generally used are 200 μm to 300 μm in thickness, and those having a screen size as large as 50 inches or above has a problem of an insufficient level of rigidity or stiffness, causing deflection, and making proper handling thereof difficult. It is also anticipated that lowering in the rigidity of the optical sheets may degrade workability in assembly of backlight devices or liquid crystal display devices, and may make it more likely to cause deflection due to heat of the backlight. This may cause disturbance in the beam condensing characteristics, and may consequently degrade the image quality.
Another possible measure may be such as thickening a base composing the prism sheet, but excessive thickness of the base makes integrated molding with the prism layer difficult.